Method in drying of paper

ABSTRACT

A method in consolidation and drying of paper. A wet web is contacted with superheated steam, whose temperature is 200-600° C. for the purpose of reducing, by heat transfer from the superheated stream, the water content of the web by evaporation of a substantial part of the water in the same. The wet web is supported and transported by means of a heat-conductive, gas-impermeable belt whose width is equal to or exceeds the width of the web, and jets of the superheated steam are directed towards the side of the web facing away from the belt.

FIELD OF THE INVENTION

The present invention relates to a method in consolidation and drying ofpaper. A wet web is contacted with superheated steam for the purpose ofreducing, by heat transfer from the superheated steam, the water contentof the web by evaporation of a substantial part of the water.

BACKGROUND ART

Drying of a web-shaped material, such as paper, usually occurs by thewet web being contacted with hot surfaces or by blowing a hot gas,preferably air, towards the web.

In cylinder drying machines, the web-shaped material is heated by heatedcylinders against which the web-shaped material is pressed by the webtension or by means of a felt or wire. The cylinders are heated by steambeing supplied to their interior and condensing when contacting thecasing of the cylinders. The condensate is evacuated continuously.

Water is preferably drawn off in free ventilated draws between thecylinders. The ventilation air, or at least the major thereof, is drawnoff from the casing which encloses the cylinder drying machine. Theamount of ventilation air drawn off is adjusted so that the moisturecontent of the exhaust air is adjusted towards a desired value, which isas high as possible in consideration of the risk of condensation etc.

Drying of so-called soft tissue occurs also on a single heated cylinder,a so-called yankee drier, in which drying occurs in combination withimpingement of hot air or combustion gases.

Drying of paper also occurs completely without contact with aheat-emitting surface in drying machines with a so-called airborne web.In these drying machines, the web floats in a fixed position above orbetween blow boxes. The air leaving the blow boxes serves to carry theweb, heat it and remove evaporated water.

The drying method affects to a very high degree the quality of the paperproduced. The quality is also affected by a plurality of otherparameters, such as how quick the paper is dried and in whichtemperature range it is treated.

Another method was already suggested in 1952 by Julian Dungler in U.S.Pat. No. 2,590,849. This method discloses a method for drying of cloth,paper and similar fibrous materials. Instead of using hot air or someother hot gas in drying, jets of superheated steam are directed at highspeed towards the goods to be dried.

In conventional drying it is not possible to increase the temperature ofthe goods to be dried above the wet temperature as long as “free water”exists. By blowing steam, a bulk temperature corresponding to thecurrent boiling point can be achieved very quickly during simultaneouscondensation of part of supplied steam.

At the beginning of a paper drying process, where air at atmosphericpressure is used, a web has a temperature of only 50-60° C. whereas,when blowing superheated steam, in steam atmosphere, the web quicklyreaches a temperature between 90 and 100° C. In addition to a reductionof the space required, this may result in an increased paper quality bythe fibres being softer and being more strongly bonded to each other.

The advantages of steam drying appear to be so obvious that it isstrange that this method did not gain a considerable market share, butsince this has not occurred, there must have been decisive difficulties,so that the advantages did not outweigh these additional problems.

OBJECTS OF THE INVENTION

A main object of the present invention is to provide a simple andspace-saving method for drying of paper.

A second object of the present invention is to provide a method ofdrying paper in an oxidation-free atmosphere.

A third object of the present invention is to provide a method for fixeddrying of paper.

A fourth object of the present invention is to provide a method fordrying of paper with superheated steam instead of e.g. air, in whichexpected quality gains can be achieved without increased costs ofinstallation and operation.

SUMMARY OF THE INVENTION

The present invention relates to a method in consolidation and drying ofpaper. A wet web is contacted with superheated steam, whose temperatureis 200-600° C., for the purpose of reducing, by heat transfer from thesuper-heated steam, the water content of the web by evaporation of asubstantial part of the water in the same.

In the suggested method, the wet web is supported and transported with aheat-conductive, gas-impermeable belt whose width is equal to or exceedsthe width of the web and jets of the superheated steam are directedtowards the side of the web facing away from the belt.

GENERAL DESCRIPTION OF THE INVENTION

Properties and quality of a dried web are largely determined by the wayof drying it. In addition to the technical conditions that must be takeninto consideration, it must also be possible to produce the producteconomically to allow it to be sold in large volumes on a competitivemarket.

According to the present invention, a wet paper web is consolidated anddried in an oxidation-free atmosphere by contacting the web withsuperheated steam, whose temperature is 200-600° C. The wet web issupported and transported with a heat-conductive, gas-impermeable beltwhose width is equal to or exceeds the width of the web and jets of thesuperheated steam are directed towards the side of the web facing awayfrom the belt. By heat-conductive belt is here and henceforth meant abelt whose thermal conductivity is so good as to function as atemperature-equalising factor in the contact with the web. The belt isconveniently made of metal, such as stainless steel, but polymers arenot excluded. By jets is meant flows with high speed and of essentiallyarbitrary cross-section, such as circular and elongate.

The method according to the invention aims at providing quicker dryingthan conventional methods, but all the same affording reasonable time,essentially exceeding the time in, for example, the nip of the impulsedrying, for consolidation of the web. Superheated steam is blown towardsthe web for a period of, for example, 0.5-5 s.

A wire may be arranged between the belt and the web, but preferably theweb is in direct contact with the belt. The belt can also be preparedfor varying adhesion, and in case of increased adhesion shrinkage in thetransverse direction can be prevented.

The drying can occur in a single step by steam of the same temperaturebeing supplied during the entire drying procedure, but it is in manycases advantageous to divide the drying into two or more steps. Then thetemperature of the steam in a first step is suitably higher than in oneor more subsequent steps.

In two-step drying, for instance the temperature of the steam in thefirst stage is selected to be between 400 and 600° C., preferablybetween 480 and 540° C., and in a second step, for example, atemperature between 300 and 400° C. is selected, preferably between 330and 370° C. The speed of impingement should be above 50 m/s, preferablybetween 80 and 120 m/s in the first step and between 60 and 120 m/s inthe second step.

Advantageously the web is preheated to 90-100° C. before drying. Thiscan conveniently take place by direct or indirect contact withessentially saturated water vapour. The preheating can be preceded by atreatment which serves to remove the air which is entrained in the wetweb. The steam for preheating is suitably taken from the drying machineas evacuated steam from one of the drying steps, in the first place thesecond step or a later step in a multistage drying machine. Theevacuated steam from the first drying step can also advantageously beused for supply to the second step or to a later step etc.

After preheating, the web can advantageously be mechanically dewateredto reduce the water content and simultaneously any remaining air can beevacuated. By this occurring at an increased temperature, the pressingwill be more efficient and less water need be evaporated during drying.In this manner, it is also possible to render the wet pressing at theend of the wet end more efficient and thus reduce the investment cost byexcluding one or more pairs of rolls. Analogously, it is advantageouslypossible to incorporate glazing in the drying machine where the web isalready surrounded by hot water vapour.

According to a variant of the invention, the drying machine can besidesbe used as a chemical reactor by chemicals which affect the propertiesof the paper being supplied together with the vapour or in some othersuitable manner in the preheating step and/or in one or more dryingsteps.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theaccompanying drawings in which

FIG. 1 is a flow chart of an embodiment of the invention;

FIG. 2 illustrates a first embodiment of a paper drying machineaccording to the invention; and

FIG. 3 illustrates a second embodiment of a paper drying machineaccording to the invention.

DESCRIPTION OF A SUGGESTED EMBODIMENT

FIG. 1 illustrates in the form of a flow chart a paper drying machine 1where a web 2 passes through a treatment device 3 consisting of apreheating step 11 and two drying steps 12, 13. In a combustion chamber17 hot flue gases are generated which are conducted to a hightemperature heat exchanger 14 and after that to a low temperature heatexchanger 15. In the high temperature heat exchanger 14, hightemperature steam is generated which is supplied to the first dryingstep 12. In the low temperature heat exchanger 15, low temperature steamis generated which is supplied to the second drying step 13. Surplussteam from the first drying step 12 is conducted to the second dryingstep 13. The surplus steam from the second drying step 13 is conductedpartly via a device 16 for saturation with water to the preheating step11 while the surplus steam from the second drying step 13, like thesteam evacuated from the preheating step 11, is removed as waste steamto be used for other purposes. Steam is recirculated in each drying step12, 13 by means of two fans 18, 19.

FIG. 2 is a schematic view of a first embodiment according to theinvention of a treatment device 3 consisting of one preheating step 11and two drying steps 12, 13. A web 2 is transported through all steps bymeans of a belt 4 of stainless steel running over two turning rolls 5and 6. The preheating step 11 is provided with a nozzle system 11 a forblowing saturated water vapour towards the web 2. Similarly, the twodrying steps are provided with nozzle systems 12 a and 13 a for blowingsuperheated water vapour towards the web 2. The nozzle systems are, forthe sake of clarity, drawn as tubing systems or the like. It goeswithout saying that they can be designed in many other ways, forinstance as blow boxes or gaps. Used water vapour together with thesteam that is being evaporated during drying is evacuated throughconduits 11 b, 12 b and 13 b.

FIG. 3 is a schematic view of a second embodiment according to theinvention of a treatment device 3 consisting of one preheating step 11and two drying steps 12, 13. A web 2 is transported through thepreheating step 11 by means of a first belt 41 of stainless steelrunning over two turning rolls 51 and 61. The web 2 is transportedthrough the two drying steps 13 by means of a second belt 42 ofstainless steel which runs over two turning rolls 52 and 62. Between thepreheating step 11 and the drying step 12 there is a device 7 formechanical dewatering, with two rolls 81 and 82 and subjacent means 9for collecting press water. Steam is supplied and evacuated in the sameway as in the embodiment according to FIG. 2.

In an exemplified embodiment of the invention, as described withreference to FIGS. 1 and 2, 25 t of paper an hour is dried in thetreatment device 3, the pulp being based on completely dry substance.The dry content of the entering web 2 is 50%. In the preheating step 11steam having a temperature of 103° C., 6.5 t/h, is supplied, and the web2 is heated to 95° C. with a dry content of about 40% duringcondensation of the major part of the steam.

In the first drying step 12, steam having a temperature of 510° C. and aspeed of 100 m/s, about 220 t/h, is made to impinge. 21 t of water anhour, at a temperature of 385° C., leaves the paper web 2, and thisquantity is supplied to the next drying step 13 while the remainder isrecirculated in the first drying step 12. The recirculating flow isheated in the high temperature heat exchanger 14 to 510° C. The drycontent of the web 2 increases in the first drying step 12 to about 71%.

In the second drying step 13, steam having a temperature of 350° C. anda speed of 100 m/s, about 290 t/h, is made to impinge. 7 t of water anhour at a temperature of 320° C. leaves the web 2, and this quantity isremoved together with the amount evacuated in the first drying step 12,21 t/h, while the remainder is recirculated in the second drying step13. The recirculating flow is heated in the low temperature heatexchanger 15 to 350° C. The dry content of the web 2 increases in thesecond drying step 13 to about 90%.

The steam evacuated from the second drying step 13 is divided into aflow which is saturated with water, in the device 16 intended therefor,and a flow of waste steam for use, for example, in heating of thebuilding or in other process steps. The saturated water vapour is usedin the preheating step 11 and the excess steam is removed therefrom.

The heat required for the recirculation flows through the hightemperature heat exchanger 14 and the low temperature heat exchanger 15is generated by combustion of oil in the combustion chamber 17. The hotflue gas from the combustion chamber 17 is conducted through the hightemperature heat exchanger 14 as well as the low temperature heatexchanger 15 in this order and then to combustion air preheaters and gascleaning systems (not shown).

ALTERNATIVE EMBODIMENTS

The invention is, of course, not restricted to the examples above andcan be varied in many ways within the scope of the appended claims.

For instance, a paper drying machine according to the suggestedembodiment can be combined with conventional drying with an airborne webor impingement of hot air. This occurs conveniently in such manner thatthe final drying takes place in a previously known manner while thefirst part of the drying is carried out according to the presentinvention. The first part of the drying can then result in the web beingsubjected to superheated steam for 0.5-2 s and leaving this drying stepwith a dry content of about 70%.

What is claimed is:
 1. A method in consolidation and drying of paper, awet web being contacted with superheated steam, at the temperature of200-600° C., for the purpose of reducing, by heat transfer from thesuperheated steam, the water content of the web by evaporating asubstantial part of the water in the same, comprising supporting andtransporting the wet web by means of a heat-conductive, gas-impermeablebelt whose width is equal to or exceeds the width of the web, anddirecting jets of the superheated steam towards the side of the webfacing away from the belt; wherein the drying occurs in at least twosteps with different temperature of the superheated steam.
 2. A methodas claimed in claim 1, wherein the web is in direct contact with thebelt.
 3. A method as claimed in claim 1, comprising supplying, in thesecond drying step or a later drying step, at least part of the steamevacuated from the first drying step.
 4. A method as claimed in claim 1,wherein the drying is preceded by a preheating step in which the web iscontacted with, or heated indirectly with, essentially saturated watervapour.
 5. A method as claimed in claim 4, wherein the web is contactedwith the heat-conductive, gas impermeable belt before the preheatingstep and is kept in contact with said belt through at least one dryingstep.
 6. A method as claimed in claim 4, wherein the web is contactedwith a heat-conductive, gas impermeable belt before the preheating stepand is kept in contact with said belt through all drying steps.
 7. Amethod as claimed in claim 4, wherein the web is preheated to atemperature between 90 and 100° C.
 8. A method as claimed in claim 4,comprising draining the web mechanically after the preheating.
 9. Amethod as claimed in claim 1, comprising blowing, in a first dryingstep, steam having a temperature between 400 and 600° C. and a speedexceeding 50 m/s, preferably having a temperature between 480 and 540°C. and a speed between 80 and 120 m/s.
 10. A method as claimed in claim1, comprising blowing, in a second drying step, steam having atemperature between 300 and 400° C. and a speed exceeding 50 m/s,preferably having a temperature between 330 and 370° C. and a speedbetween 60 and 120 m/s.
 11. A method as claimed in claim 3, comprisingsupplying, in the preheating step, at least part of the steam evacuatedfrom a drying step, preferably from the second drying step or a laterdrying step.
 12. A method as claimed in claim 4, comprising supplying,in the preheating step, at least part of the steam evacuated from adrying step, from at least one of the second drying step or a laterdrying step.
 13. A method as claimed in claim 1, comprising addingchemicals in at least one of the drying steps and/or the preheatingstep.
 14. A method as claimed in claim 1, comprising glazing the paperin one of the drying steps according to the invention.
 15. A method asclaimed in claim 2, wherein the drying is preceded by a preheating stepin which the web is contacted with, or heated indirectly with,essentially saturated water vapour.
 16. A method as claimed in claim 10,comprising supplying, in the preheating step, at least part of the steamevacuated from a drying step, preferably from the second drying step ora later drying step.
 17. A method as claimed in claim 5, comprisingdraining the web mechanically after the preheating.
 18. A method asclaimed in claim 6, comprising draining the web mechanically after thepreheating.
 19. A method as claimed in claim 7, comprising draining theweb mechanically after the preheating.
 20. A method as claimed in claim4, comprising blowing, in a first drying step, steam having atemperature between 400 and 600° C. and a speed exceeding 50 m/s,preferably having a temperature between 480 and 540° C. and a speedbetween 80 and 120 m/s.
 21. A method as claimed in claim 5, comprisingblowing, in a first drying step, steam having a temperature between 400and 600° C. and a speed exceeding 50 m/s, preferably having atemperature between 480 and 540° C. and a speed between 80 and 120 m/s.22. A method as claimed in claim 6, comprising blowing, in a firstdrying step, steam having a temperature between 400 and 600° C. and aspeed exceeding 50 m/s, preferably having a temperature between 480 and540° C. and a speed between 80 and 120 m/s.
 23. A method as claimed inclaim 7, comprising blowing, in a first drying step, steam having atemperature between 400 and 600° C. and a speed exceeding 50 m/s,preferably having a temperature between 480 and 540° C. and a speedbetween 80 and 120 m/s.
 24. A method as claimed in claim 8, comprisingblowing, in a first drying step, steam having a temperature between 400and 600° C. and a speed exceeding 50 m/s, preferably having atemperature between 480 and 540° C. and a speed between 80 and 120 m/s.25. A method as claimed in claim 4, comprising blowing, in a seconddrying step, steam having a temperature between 300 and 400° C. and aspeed exceeding 50 m/s, preferably having a temperature between 330 and370° C. and a speed between 60 and 120 m/s.
 26. A method as claimed inclaim 5, comprising blowing, in a second drying step, steam having atemperature between 300 and 400° C. and a speed exceeding 50 m/s,preferably having a temperature between 330 and 370° C. and a speedbetween 60 and 120 m/s.
 27. A method as claimed in claim 6, comprisingblowing, in a second drying step, steam having a temperature between 300and 400° C. and a speed exceeding 50 m/s, preferably having atemperature between 330 and 370° C. and a speed between 60 and 120 m/s.28. A method as claimed in claim 7, comprising blowing, in a seconddrying step, steam having a temperature between 300 and 400° C. and aspeed exceeding 50 m/s, preferably having a temperature between 330 and370° C. and a speed between 60 and 120 m/s.
 29. A method as claimed inclaim 8, comprising blowing, in a second drying step, steam having atemperature between 300 and 400° C. and a speed exceeding 50 m/s,preferably having a temperature between 330 and 370° C. and a speedbetween 60 and 120 m/s.
 30. A method as claimed in claim 9, comprisingblowing, in a second drying step, steam having a temperature between 300and 400° C. and a speed exceeding 50 m/s, preferably having atemperature between 330 and 370° C. and a speed between 60 and 120 m/s.31. A method as claimed in claim 9, comprising supplying, in the seconddrying step or a later drying step, at least part of the steam evacuatedfrom the first drying step.
 32. A method as claimed in claim 10,comprising supplying, in the second drying step or a later drying step,at least part of the steam evacuated from the first drying step.
 33. Amethod as claimed in claim 9, comprising supplying, in the preheatingstep, at least part of the steam evacuated from a drying step,preferably from the second drying step or a later drying step.
 34. Amethod in consolidation and drying of paper, a wet web being contactedwith superheated steam, at the temperature of 200-600° C., for thepurpose of reducing, by heat transfer from the superheated steam, thewater content of the web by evaporating a substantial part of the waterin the same, comprising supporting and transporting the wet web by meansof a heat-conductive, gas-impermeable belt whose width is equal to orexceeds the width of the web, and directing jets of the superheatedsteam towards the side of the web facing away from the belt, wherein thedrying is preceded by a preheating step in which the web is contactedwith, or heated indirectly with, essentially saturated water vapour, andwherein the web is preheated to a temperature between 90 and 100° C. 35.A method in consolidation and drying of paper, a wet web being contactedwith superheated steam, at the temperature of 200-600° C., for thepurpose of reducing, by heat transfer from the superheated steam, thewater content of the web by evaporating a substantial part of the waterin the same, comprising supporting and transporting the wet web by meansof a heat-conductive, gas-impermeable belt whose width is equal to orexceeds the width of the web, directing jets of the superheated steamtowards the side of the web facing away from the belt, and blowing, in asecond drying step, steam having a temperature between 300 and 400° C.and a speed exceeding 50 m/s, preferably having a temperature between330 and 370° C. and a speed between 60 and 120 m/s, wherein the dryingis preceded by a preheating step in which the web is contacted with, orheated indirectly with, essentially saturated water vapour.